Method of producing sodium dicyanamide

ABSTRACT

A process is described for preparing sodium dicyanamide, in which cyanamide is reacted simultaneously with sodium hydroxide solution and cyanogen chloride in aqueous solution at temperatures of 20 to 100° and a pH of 7.0 to 10.0. By means of the inventive process it is possible, starting from raw materials which are available in technical-grade quality, to prepare sodium dicyanamide in good yields of 75 to 95% and very high purities of up to 100% in a very environmentally friendly manner, for which reason this process is particularly highly suitable for the industrial scale.

DESCRIPTION

The present invention relates to a preferably continuous process forpreparing sodium dicyanamide which is suitable in particular for theindustrial scale.

Sodium dicyanamide is used in large amounts for preparing biocidalactive compounds in the sanitary and healthcare sectors and fordisinfection in food production.

According to the prior art, various methods are used for preparingsodium dicyanamide. Thus, for example, CA Vol. 109, 218 568 (1988)describes the reaction of ammonia with cyanogen chloride and metalhydroxides at 20 to 30° C. This achieves a conversion rate of 90.5% witha purity of 94.3%. However, this procedure is associated with somefundamental disadvantages. The low temperature level in the highlyexothermic reaction requires the use of cooling brine and thus expensiveelectrical cooling energy. Since the space/time conversion rate isdetermined primarily by the effectiveness of heat removal and, in thecase of this process, furthermore a relatively large amount of heat ofreaction is liberated, very high production costs because of therelatively large apparatuses and poor heat balance must be expected. Inaddition, roughly twice the amount of cyanogen chloride and sodiumhydroxide solution are consumed per mole of sodium dicyanamide in orderto prepare the intermediate cyanamide. In addition, an equimolar amountof contaminated sodium chloride is produced, which must be disposed ofin a very costly manner, which appears problematic with respect toenvironmental aspects.

A general problem in the preparation of sodium dicyanamide is removingthe by-product sodium chloride, since the two compounds are sodium saltswith good solubility in water. A process in which twice the amount ofsodium chloride is produced in a mixture with the target product thusalso leads to considerably impaired yield of pure sodium dicyanamide.

The sodium dicyanamide purity required by active compound manufacturersis customarily at least 97% by weight, frequently even at least 98% byweight, which is not achieved by the process according to CA 109; 210568 (1988).

A similar process is described in CA Vol. 110; 138 089 (1989), in whicha solution of cyanogen chloride in benzene is charged and ammonia isadded. The amount of cyanogen chloride charged and the use of organicsolvent, in this case the carcinogenic benzene, make this processimpracticable on the industrial scale.

Canadian Patent 956 081 describes an alternative synthetic pathway forsodium dicyanamide, starting from cyanamide, sodium cyanide andchlorine, with addition of sodium hydroxide solution.

From the reaction equation it is clear that, in this process also, twomoles of sodium chloride are formed per mole of sodium dicyanamide, withthe result that the separation problems already described above occurwith adverse consequences on product purity and yield. Although theprocess gives conversion rates of >96%, isolated yields of 73 to 78% areobtained at a purity of 73 to 86% by weight. This reflects thedifficulty of the sodium chloride removal which has been discussed. Fromthe aspect of raw material and disposal costs, this process is alsodisadvantageous, since the raw materials sodium cyanide and chlorineused are considerably more expensive than cyanogen chloride and sodiumhydroxide solution.

The object underlying the present invention, therefore, was to develop aprocess for preparing sodium dicyanamide which does not have saiddisadvantages corresponding to the prior art, but which permits sodiumdicyanamide to be prepared from inexpensive raw materials which areavailable on an industrial scale and thus meets the stringentrequirements of safety and product purity.

This object is achieved according to the invention by cyanamide beingreacted simultaneously with sodium is hydroxide solution and cyanogenchloride in aqueous solution at temperatures of 20 to 100° C. and a pHof 7.0 to 10.0.

It has surprisingly been found here that an extremely pure sodiumdicyanamide can be obtained in very good yields even with the use oftechnical-grade raw materials. This was surprising because the processcan be carried out even at elevated temperature conditions withoutdisadvantageous effects on purity of the product and selectivity of thereaction and it is known to those skilled in the art that under theclaimed reaction conditions, usually cyanamide is very rapidly dimerizedand cyanogen chloride is very rapidly hydrolyzed to cyanate by sodiumhydroxide solution. Against this background, among those skilled in theart there was the prejudice for keeping the reaction temperature as lowas possible for reactions with cyanogen chloride in aqueous alkalimedia.

In the process according to the present invention, cyanamide in aqueoussolution is simultaneously reacted with sodium hydroxide solution andcyanogen chloride in. The choice of cyanamide as raw material means thatin the reaction only 1 equivalent of sodium chloride is produced asby-product. Preferably, cyanamide is used in the form of a 20 to 60% byweight aqueous solution, in particular in the commercially conventionalconcentration of 50% by weight (SKW cyanamide L500) and the sodiumhydroxide solution is used as 10 to 50% by weight aqueous solution, inparticular 20 to 30% by weight solution. If the concentration ofcyanamide is decreased, it is advantageous to increase the content ofsodium hydroxide solution, and vice versa.

The product concentration can be controlled as desired by theconcentration of the raw materials cyanamide and sodium hydroxidesolution. Thus, preferably, raw material concentrations are used fromwhich, without evaporation or dilution operations, a productconcentration results at which the reaction product remains completelyin solution and from which, during the crystallization, sodiumdicyanamide crystallizes out to the greatest possible extent, but thestoichiometric by-product sodium chloride does not yet crystallize out.However, in principle, it is possible to choose the concentrations ofthese reaction components independently and as desired, if theabovementioned advantage of a direct crystallization of the product isnot intended or if the reaction mixture is set later to the desiredconcentration by concentration or dilution.

Cyanogen chloride may preferably be used according to the inventiveprocess as a technical-grade gas, which is of critical importance bothfor the economic efficiency and for plant safety. Firstly, in contrastto the use in condensed or dissolved form, the holdup in a plant can bekept extremely low, even for an industrial scale production, and thushazard to personnel and environment can be virtually excluded. In thecase of gaseous cyanogen chloride, in contrast to condensed or highlyconcentrated solutions, there is also not the risk of a spontaneoushighly exothermic trimerization.

Technical-grade cyanogen chloride also comprises minor components in therange from 3 to 8% by volume, for example carbon dioxide or chlorine. Inthis case also, it has surprisingly been found that these minorcomponents lead to completely harmless products, which are not presentin the isolated sodium dicyanamide.

It is considered an important advantage of the inventive process thatthe reaction, in contrast to the prior art, can be carried out even atelevated temperature, in particular at 40 to 80° C., without sidereactions occurring to an increased extent in this case. This hascritical consequences from the ecological and process-engineeringaspects. Firstly, the great exothermy of the reaction is utilized toheat the reaction parameters added cold; secondly, the excess heat canbe simply removed via a heat exchanger using cold water. In theprocesses corresponding to the prior art, the temperature gradient ofreaction solution to cooling water is not sufficient for an economicprocedure, so that electrical cooling energy must be employed.

When the inventive reaction is carried out, it has proved to beparticularly advantageous to control the amounts of reactionparticipants in a targeted manner. This is because it has been found inthis case that the reaction and also the following process steps proceedoptimally if a defined stoichiometric ratio is maintained during eachphase of the metering. The metering of sodium hydroxide solution andcyanamide is preferably implemented by a rate measurement, as a functionof the concentration of these raw materials. In this case the ratio ofthe reactants is preferably set in such a manner that 2.0 to 2.4 mol,preferably 2.1 to 2.2 mol, of sodium hydroxide are used per mole ofcyanamide. The exact ratio is dependent on the purity of the cyanogenchloride used.

Exact and reliable rate-controlled metering of gaseous industrial-gradecyanogen chloride is extremely difficult from the metrological aspect.Conventional apparatuses for flow metering are of little use inpractice. For this reason, preferably, cyanogen chloride is added underpH control in such a manner that in the reaction solution a pH of 7.0 to10.0, preferably 7.0 to 8.5, is maintained. At this pH, the reactionpartners react immediately in the desired manner. At a lower pH there isthe risk that the extremely toxic cyanogen chloride is not reactedcompletely to exhaustion and is released during work-up. At higher pHs,side reactions such as hydrolysis of cyanogen chloride and dimerizationof cyanamide occur to an increased extent. Generally, cyanogen chlorideis used in an equimolar, or approximately equimolar, ratio based on thecyanamide used. According to a preferred embodiment, the raw materialsare metered in separately simultaneously to a reactor with good mixingand the reaction solution is taken off continuously. Although priormixing of cyanamide and sodium hydroxide solution, or cyanamide andcyanogen chloride, is possible in principle, it has disadvantages withrespect to product quality or process safety.

By means of the simultaneous metering of the raw materials into areactor (for example a residence-time reactor) with good mixing, thesame concentration, temperature and pH conditions are always presentduring the entire course of the reaction. This leads to two importantbeneficial effects, that is to say, firstly, minimizing unwantedby-products, for example dicyanodiamide and sodium N-cyanoisourea, whichinhibit a clean selective crystallization of the product and arethemselves difficult to remove. Secondly, as a result, the dimensions ofthe reactor, the heat exchanger and other apparatuses can be reduced,which leads to considerable savings in capital costs and maintenance.Suitable reactors are, for example, recirculation reactors equipped withstatic mixers or mixing nozzles or tubular reactors havinggas-introduction agitators.

Following the reaction, sodium dicyanamide can be crystallized out invery high purity from the hot reaction solution directly by targetedcrystallization either batchwise or via a controlled cooling curve or acontinuous crystallizer. If the procedure starts from technical-graderaw materials, the reaction solution can still contain small amounts ofdiscoloring impurities, which are generally not of high-molecular-weightnature.

According to a preferred embodiment, this discoloration can beeffectively eliminated by treatment with activated carbon, even in verylow amounts. For this, the product solution is admixed hot with 0.1 to 5g of activated carbon per liter and the carbon is removed again in aconventional manner before the crystallization. Alternatively, thesolution can also be run through an activated carbon bed or filtersprepared with activated carbon.

An important advantage of the present invention must be considered to bethe fact that, despite a high concentration, the product is maintaineddissolved in the reaction solution, or is brought completely intosolution by further heating to, for example, 60 to 100° C.

Starting from this solution, by controlled cooling, selectivecrystallization of sodium dicyanamide can be achieved, while thestoichiometric by-product sodium chloride remains in solution.Crystallization proceeds uniformly without formation of cocrystals orinclusions, so that complex recrystallization of the product or otherwork-up is not required. Selective crystallization of sodium dicyanamideis also possible by controlled continuous addition of hot sodiumdicyanamide solution to a cooled product suspension, or by concentrationfrom dilute solutions, if appropriate with simultaneous cooling. It iscritical in this case that the solution does not fall below thesolubility curve of sodium chloride.

Sodium dicyanamide is isolated then in a customary manner by filtration,with adherent residual mother liquor being able to be removed by carefulwashing with water.

Using the inventive process it is possible, starting from raw materialsavailable in technical-grade quality, to prepare sodium dicyanamide in avery environmentally friendly manner in good yields of approximately 75to 95% and very high purities of up to 100%, for which reason thisprocess is particularly highly suitable for the industrial scale.

The examples below are intended to illustrate the invention in moredetail.

EXAMPLES Example 1

A 2.5 l recirculation reactor equipped with temperature and pHmeasurement, heat exchanger and metering system having a static mixerwas operated continuously. The metering system consisted of a glass tubein the reactor circuit having separate ports for cyanamide, sodiumhydroxide solution and cyanogen chloride. Immediately downstream ofthese metering ports was a mixing section (Ø 1.6 cm, length 5 cm)containing Sulzer mixing elements. The mean circulation rate was 14l/min. At the start, the reactor was charged with water at 60° C. andthen metering of the raw materials was started.

The following amounts were added per hour via peristaltic pumps:

Raw material Concentration Amount/h Mol/h Cyanamide 50.1%   515 g 6.13Sodium hydroxide 28.0% 1 857 g 13.00 solution

Technical-grade cyanogen chloride was introduced simultaneously in thegaseous state in such a manner that a pH of 7.5 to 8.0 was maintained.The consumption was determined at 370 g/h (6.02 mol/h) based on puresubstance. At this metering rate a mean residence time of 1 hour 35minutes in the reactor resulted. The heat exchanger was charged withenough cooling water to set a reaction temperature of 70 to 75° C. Thevolumes metered in were taken off via a free overflow and passed througha bed containing powdered activated carbon (10 g per 15 l of reactionsolution) into a buffer vessel. The buffer vessel was kept at 75° C.during charging and then cooled to 0° C. in the course of 7.5 hoursusing a linear cooling curve. By alternately filling two buffer vessels,the reaction could be conducted continuously. After crystallization, theproduct was filtered off using suction and washed with 500 ml of icewater per kg of filter cake. Sodium dicyanamide was obtained at a purityof 100% without detectable contamination with dicyandiamide or sodiumN-cyanoisourea, and with a chloride content of 0.2%. The APHA value(discoloration) of a 10% strength solution in water was 10. The yieldisolated was 76%, based on cyanamide used.

Example 2

In a similar manner to Example 1, sodium dicyanamide was preparedcontinuously in the apparatus described except that the reactionsolution was not filtered through activated carbon. The sodiumdicyanamide isolated had a purity of 99% without detectablecontamination with dicyandiamide or sodium N-cyanoisourea, and with achloride content of 0.6%. The APHA value (discoloration) of a 10%strength solution in water was 55.

Example 3

A recirculation reactor system having a total volume of 4.2 l,consisting of a double-jacketed glass vessel, a diaphragm recirculationpump and a metering system for the raw materials was operatedcontinuously to prepare sodium dicyanamide. The metering systemconsisted of a driving jet nozzle operated with reaction solution, whichdrew in gaseous cyanogen chloride and mixed it with the reactionpartners cyanamide and sodium hydroxide solution which were metered inimmediately at the nozzle exit via peristaltic pumps. Via the lid of theglass vessel, a free overflow led into a buffer vessel heated to 70° C.In the glass vessel temperature and pH were monitored and the meteringof the raw materials was controlled accordingly. The amount circulatedwas taken off from the bottom of the reactor and brought to the pressureof 1.4 bar which was required for the driving jet by means of adiaphragm pump. To even out the pressure, an equilibration vessel wasconnected between pump and driving jet nozzle. This experimentalarrangement was operated with the following parameters:

Circulation rate: 210 l/hour Constant molar ratio of sodium hydroxide tocyanamide = 2.18 Concentration of cyanamide: 50.0% Concentration ofsodium 28.0% hydroxide solution: Temperature in the recirculation 70 to75° C. reactor: Nozzle pressure: 1.4 bar pH: 7.2-8.0 Metering rates:Cyanamide: 925 g/hour = 11.01 mol/hour Sodium hydroxide solution: 3430g/hour = 24.01 mol/hour Cyanogen chloride (96% pure): 705 g/hour = 11.01mol/hour

At the start, the recirculation reactor was filled with water at 70° C.and the circulation rate was set. The raw material metering was thenstarted, with the cyanogen chloride rate being controlled in such amanner that the internal temperature, under external cooling withcooling water (18° C.), remained in the range 70 to 75° C. The liquidcomponents cyanamide and sodium hydroxide solution were fed at aconstant molar ratio via controllable peristaltic pumps under pHmonitoring. Via the free overflow, the product solution was collected inthe buffer vessel and after it was filled, drained off into an agitatedcrystallization vessel. The solution was cooled to 0° C. in the courseof 4 hours via a linear cooling curve and the product was isolated byfiltration on a vacuum filter funnel. The product was washed with 500 mlof ice water per kg of filter cake and dried under reduced pressure at6° C. In this manner, 803 g (=82% of theory) of sodium dicyanamide at apurity of 98.5% were obtained per hour. The sole impurity presentat >0.1% was sodium chloride at 1.3%.

Example 4

In a similar manner to Example 3, a sodium dicyanamide solution wasprepared continuously, but, downstream of the free overflow, this wasrun out of the reactor system directly into a product suspension chargedinto the buffer vessel and cooled to 0° C. After the buffer vessel wasfilled, this was emptied down to a remainder of approximately 1 literand filled again without interruption. The sodium dicyanamide suspensiontake off was filtered in the usual manner, the filter cake was washedand dried.

814 g (=83% of theory) of sodium dicyanamide at a purity of 98.3% wereobtained per hour.

Example 5

In a similar manner to Example 3, sodium dicyanamide was prepared, butthe filter cake was not washed during the product isolation. 920 g/hour(=94% of theory) of sodium dicyanamide at a purity of 94.8% wereobtained. The product comprised 4.8% of sodium chloride and 0.18% ofsodium N-cyanoisourea.

Example 6

Sodium dicyanamide was prepared in accordance with Example 3, exceptthat in the reaction part a temperature of 50 to 55° C. was maintained,whereas in the buffer vessel a temperature of 900° C. was set.

Because of the low temperature level for the same cooling area, in thisexperiment the following amounts could be metered per hour:

-   -   823 g (9.8 mol) of cyanamide (50% strength)    -   3050 g (20.4 mol) of sodium hydroxide solution (28% strength)    -   608 g (9.5 mol) of cyanogen chloride (96% strength)

During the reaction, crystals had already formed, which redissolved inthe heated buffer vessel, however. After the corresponding work-up, 690g/hour (79% of theory) of sodium dicyanamide at a purity of 97.6% wereobtained.

Example 7

Sodium dicyanamide was prepared continuously in accordance with Example6, except that a 15% by weight sodium hydroxide solution was used. Thefollowing amounts were metered per hour in this case:

-   -   820 g (9.8 mol) of cyanamide (50% strength)    -   5700 g (20.4 mol) of sodium hydroxide solution (15% strength)    -   600 g (9.4 mol) of cyanogen chloride (96% strength)

A reddish-brown solution was obtained which was run through an activatedcarbon bed (1 g per liter of solution) at 55° C., at a residence time of10 min. The filtered solution was then evaporated under reduced pressure(approximately 200 mbar) until a thin crystal slurry was formed at 60°C. This slurry was cooled in a linear manner to 0° C. in the course of 3hours and the product isolated in the usual way. 730 g (87% of theory)of sodium dicyanamide at a purity of 97.2% were obtained per hour. Thesodium chloride content in the product was 2.4%.

1. A process for preparing sodium dicyanamide comprising simultaneouslyreacting cyanamide in aqueous solution with a sodium hydroxide solutionand cyanogen chloride, at a temperature of from 40° C. to 100° C., andat a pH of from 7.0 to 10.0.
 2. The process as claimed in claim 1,wherein cyanamide is used as a 20 to 60% by weight aqueous solution. 3.The process as claimed in claim 1, wherein the sodium hydroxide solutionis used as a 10% to 50% by weight aqueous solution.
 4. The process asclaimed in claim 1, wherein cyanogen chloride is used in the gaseousstate and in technical grade quality.
 5. The process as claimed in claim1, wherein the reaction temperature is 50 to 80° C.
 6. The process asclaimed in claim 1, wherein the pH is set at from 7.0 to 8.5.
 7. Theprocess as claimed in claim 1, wherein 2.0 to 2.4 mol of sodiumhydroxide are used per mole of cyanamide.
 8. The process as claimed inclaim 1, wherein the cyanogen chloride is used at an equimolar, orapproximately equimolar, ratio based on the cyanamide used.
 9. Theprocess as claimed in claim 1, wherein the cyanamide, sodium hydroxideand cyanogen chloride are metered to a reactor separately butsimultaneously while mixing, continuously and reaction solution isremoved continuously.
 10. The process as claimed in claim 1 comprisingmetering the cyanogen chloride while maintaining a constant pH.
 11. Theprocess as claimed in claim 1 wherein cyanamide, sodium hydroxide andcyanogen chloride are used at concentrations such that the sodiumdicyanamide produced remains completely in solution at the reactiontemperature used.
 12. The process as claimed in claim 1, comprisingcrystallizing the sodium dicyanamide out of the reaction solution andseparating it therefrom.
 13. The process as claimed in claim 12comprising treating the reaction solution with activated carbon beforecrystallizing the sodium dicyanamide.
 14. The process as claimed inclaim 13, wherein the reaction is treated with 0.1 g to 5.0 g ofactivated carbon per liter of solution.
 15. The process of claim 2,wherein the sodium hydroxide solution is used as a 10% to 50% by weightsolution.
 16. The process of claim 2, wherein the sodium hydroxidesolution is used as a 20% to 30% by weight solution.
 17. The process ofclaim 12, comprising separating said sodium dicyanamide by filtration.